1. Field of the Invention
The invention relates generally to a control over a shift-position changing mechanism, and, more specifically, to a control over a shift-position changing mechanism, which provides fail-safe protection if an erroneous determination on a shift position is made when the state of a shift lever is changed.
2. Description of the Related Art
There is a shift-position changing mechanism, which changes shift positions of an automatic transmission in response to an operation of a shift lever performed by a driver, and which is provided with an electric motor (for example, a direct-current motor) as a power source for changing the shift positions.
With such shift-position changing mechanism, the shift lever and the shift-position changing mechanism need not be mechanically connected to each other, unlike a common changing mechanism that directly changes shift positions of an automatic transmission using an operational force applied to a shift lever by a driver. This eliminates the limitation on the layout of components in a vehicle, thereby providing greater flexibility to the vehicle design. In addition, the shift-position changing mechanism thus structured is more easily fitted to the vehicle.
The shift-position changing mechanism and the shift lever need not be mechanically connected to each other. A shift position command signal issued in response to the operation of the shift lever is transmitted to the shift-position changing mechanism, and the shift positions are changed based on the shift position command signal. It is therefore necessary to accurately detect the shift position command signal issued in response to the operation of the shift lever. There have been employed contactless detection methods such as a method for detecting the position, to which the shift lever has been operated, based on a change in the magnetic field caused due to the operation of the shift lever, using a magnetic-field detection sensor.
For example, Japanese Patent Application Publication No. 10-24749 (JP-A-10-24749) descries a shift-position changing device for a vehicle, which contactlessly detects the position of a shift lever, which needs minimum number of members or components, which controls a transmission without a mechanical connection between the shift-position changing device and the transmission, and which makes it possible to simplify a ducting of the shift lever. In the shift-position changing device, the shift lever is connected to a control lug having a signal surface, the control lug corresponds to a sensor, and the sensor transmits a signal corresponding to the position of the shift lever to a control unit for the transmission.
With the shift-position changing device described above, the signal transmitted from the control lug is detected, appropriately processed, and is provided to a control system for the transmission.
However, when the position, to which the shift lever has been operated, is detected using, for example, a magnetic-field detection sensor, based on a change in the magnetic field caused due to the operation of the shift lever, if the shift-position changing device is influenced by an externally-caused change in the magnetic field, which is different from a change in the magnetic field due to the operation of the shift lever, the shift-position changing device sometimes makes an erroneous determination on the shift position indicated by a command signal issued in response to the operation of the shift lever. However, the shift-position changing device described above is configured without taking such erroneous determination into account.
Therefore, there is a likelihood that the shift-position changing device described above will erroneously regard the shift position at which the vehicle is not able to run, for example, Park or Neutral, as the shift position at which the vehicle is able to run, for example, Drive or Reverse. Conversely, there is a likelihood that the shift position changing device will erroneously regard the shift position at which the vehicle is above to run as the shift position at which the vehicle is not able to run. Accordingly, there is a likelihood that the shift position is changed to a shift position at which the vehicle behaves in a manner that does not reflect the driver's intention.